Presently there exist a wide variety of non-contact proximity detectors. The vast majority of these are electrical in nature, using either a change in inductance or capacitance introduced by the presence of a suitable target to allow detection of the target. Other sensors employ optical means using the reflection of light from a target or similar means employing the transmission of a light beam beyond the sensor and subsequent detection of a changed beam.
Most existing fibre optic proximity detectors employ a transmission and detection fibre and rely on light reflection external to the sensor to provide the indication of target presence. While suitable for a wide variety of tasks, these sensors are not appropriate in dirty conditions where the emitting or sensing portion of the sensor may be obscured by dust, dirt, or other contaminants.
In aircraft applications, inductive proximity sensing is commonly used. The sensors typically employ a two or three wire electrical connection to an interrogation system designed to query the sensor and determine its inductance, and from that infer whether a target is ‘near’ or ‘far’. While the systems in practice are reliable and proven, when a system incorporating a large quantity of sensors is envisaged, the sheer number of electrical connections required can become large. In addition, significant effort needs to be expended to ensure proper protection from high intensity radiated fields, lightning strikes, and wire length limitations.
As aircraft construction moves from predominantly metallic construction materials (such as aluminum) to more organic matrix composite construction, significant consideration needs to be given to the overall lack of shielding and lightning protection that was once afforded by having a low resistance, conductive aircraft surrounding all electrical elements. One approach to address this issue is to transition the electrical proximity sensing system to an all optical fibre system.
The present art of fibre optic proximity sensing includes two principal types of sensor—an external reflectance type sensor, where light is transmitted from one fibre and a target mirror reflects that light into a neighbouring detection fibre in the ‘near’ condition, and a magnetic sensor employing the Faraday effect. The magnetic sensor employs two fibres but utilizes a magnet as the target to change the polarization of light that passes through the sensor from one fibre to the other. This sensor is potentially viable for aircraft applications as there is no external light emitting component. However, the requirement that each sensor have two fibres is onerous in large systems and would require large amounts of fibre and electronics for querying each sensor.